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https://doi.org/10.1130/G45188.1 Manuscript received 23 May 2018 Revised manuscript received 27 July 2018 Manuscript accepted 16 August 2018 © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 25 September 2018 Human-caused avulsion in the Rhine-Meuse delta before historic embankment (The Netherlands) Harm Jan Pierik1,*, Esther Stouthamer1, Tim Schuring1, and Kim M. Cohen1,2,3 1Department of Physical Geography, Faculty of Geosciences, Utrecht University, P.O. Box 80.115, 3508 TC, Utrecht, The Netherlands 2Department of Applied Geology and Geophysics, Deltares, P.O. Box 85.467, 3508 AL, Utrecht, The Netherlands 3TNO Geological Survey of the Netherlands, P.O. Box 80.015, 3508 TA, Utrecht, The Netherlands ABSTRACT human impact (Pierik and van Lanen, 2017). The new Lek and Hollandse Although the shifting of deltaic river branches (avulsion) is a IJssel (HIJ) Rhine branches formed during this period, crossing 30 km natural process that has become increasingly influenced by humans, of freshwater peatland to connect to the tidal channels of the Old Meuse the impact of early human activities as a driver of avulsion success estuary. This caused the abandonment of the Old Rhine course (Fig. 1). has remained poorly explored. This study demonstrates how two These avulsions are excellent cases to study human impact on avulsion important avulsions in the downstream part of the Rhine-Meuse because they occurred in a data-dense area, and they allow the different delta, The Netherlands, were stimulated by human activities in the stages of the avulsion process to be studied. Their record is well preserved first millennium CE, before historic embankment constrained the because early in the second millennium, the new rivers were embanked, river courses. Peatland reclamation induced land subsidence in the limiting lateral reworking. The boundary conditions—climate, sediment lower delta. This effect, together with a human-induced increase in supply, and flooding regime, as well as human presence—are well known. suspended fluvial sediments and tidal backwater effects, allowed for a Remarkably, the Lek and HIJ were the first channels in 3000 yr that gradual ingression of tidal creek channels and progradation of fluvial managed to cross an extensive alder peat swamp. While Rhine floodwaters crevasse channels into human-occupied and drained peatlands, where had fed these swamps, older crevasse splays ran dead into the swamp, they eventually connected. We reconstructed the initial situation and implying that vegetation and substrate reduced floodwater flow velocities, identified the feedback loops among overbank sedimentation, tidal inhibiting the crevasse channels to develop into avulsion belts (Makaske incursion, and land drainage subsidence that led to avulsion success. et al., 2007). This situation changed from the Late Iron Age onward (250 The processes and feedbacks resulting from human activities are BCE), when suspended sediment delivery from the upstream river basin generic and hence relevant to many other deltas today where human- increased as a result of hinterland deforestation (Erkens et al., 2011), induced subsidence results in tidal ingression, potentially connecting while at the same time tidal ingression from the downstream direction to rivers and causing unexpected avulsions. occurred due to peatland drainage and reclamation (Pierik et al., 2017a; de Haas et al., 2018). These human-induced developments have been INTRODUCTION studied separately, but the ways in which they interacted, and the degree Avulsion, the shift of a river course, takes place in multiple phases. to which they led to lower delta avulsion are not known. The preconditioning phase occurs before the river shifts its course; dur- Here, we show for the first time how human-induced upstream and ing this phase, factors such as sediment supply and sea-level rise create downstream factors interacted to set the stage for successful avulsion the necessary conditions for eventual flow diversion (e.g., Makaske et al., in a low-gradient unembanked delta. We mapped and dated the stages 2012). Many modern fluvio-deltaic environments where avulsion takes of downstream tidal ingressions, upstream crevasse splay progradation, place have been increasingly affected by humans over the last millennia. and peatland reclamation. Furthermore, we identified the drivers and Humans are known to have deliberately triggered avulsion directly, by feedbacks that eventually led to the successful avulsion. Our case shows constructing canals and dams (e.g., Heyvaert and Walstra, 2016), but less the effects of human impact on avulsion and serves to illuminate poten- is known about unintended avulsion resulting from human impacts in tial future avulsion in other populated deltas that would have substantial the delta plain and in the hinterland before the avulsion is actually trig- socioeconomic impacts. gered. This is despite the potential importance and impact of avulsion on densely populated deltas worldwide. These areas have low topographical MATERIALS AND METHODS gradients, and, during the preconditioning phase, small human-induced The HIJ and Lek avulsion cases were traced by using an extensive changes in sedimentation or subsidence may already lead to critical shifts data set containing the age and position of channel belts and their natural in the topographically favorable pathway through a delta. levees, crevasse splays, and tidal creek landforms in the Rhine-Meuse Here, we present a case of two avulsions in the lower Rhine delta delta (e.g., Berendsen and Stouthamer, 2000). We expanded this rich data in the first millennium CE, a period during which population density set by sampling and 14C dating the top of peat directly below overbank increased, and the natural delta environment became more affected by deposits at multiple locations along the HIJ and Lek branches (see the *E-mail: [email protected] CITATION: Pierik, H.J., Stouthamer, E., Schuring, T., and Cohen, K.M., 2018, Human-caused avulsion in the Rhine-Meuse delta before historic embankment (The Netherlands): Geology, v. 46, p. 935–938, https:// doi.org/10.1130/G45188.1 Geological Society of America | GEOLOGY | Volume 46 | Number 11 | www.gsapubs.org 935 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/46/11/935/4527130/935.pdf by guest on 26 September 2021 5°W 0° 5°E 10°E yrs BCE/CE A -200 0 200 400 600 800 1000 1200 North sea DI M3(22,23) 17|GrA-62969: 2 CE±34 N M1/M2 Hollandse IJssel (HIJ) 22|Ua-37219: 50 CE±37 5° 5 UI 17 23|GrA-62970: 113 CE±41 UK Embankment NL M1/M2 Germany DI M4/M5 19|GrA-64635: 24 CE±34 °N Lek 0 27|GrA-62968: 324 CE±44 5 UI 19 27 00 B Old Rhine estuary North 600 4 Sea Old Rhine 0 0 HIJ parent channel 00 17 45 M0 M3 22 23 Lek M2 00 19 0 0 27 44 10 km to the sea D Legend archeological dates fluvial clay M4 radiocarbon dates tidal clay Roman engineering (culvert) fen and swamp peatland M1 older channel belt bog peatland 0 new channel belt 0 M5 beach-barrier ridges 052.5 10 300 older channel belts, levees, 4 Old Meuse and splays Kilometers 8000090000 100000 110000 120000 130000 140000 Figure 1. A: Time line of stage development of Hollandse IJssel (HIJ) and Lek avulsions, highlighting initiation phases at downstream (DI) and upstream (UI) locations. Main supporting 14C dates (±1σ) for samples from top of peat below river clay are indicated with black bars. M0–M5 indicate different tidal creeks from Old Meuse estuary; white rectangles indicate occurrence of rectangular creek networks. White stars indicate presumed tidal-fluvial connection moments and locations based on orientation and intersection of adjacent crevasse splays. Yellow stars indicate UI crevasse-avulsion initiation and position. B: Surficial geological map of study area showing old river courses (dark blue) and new courses (light blue; Cohen et al., 2012), flood-basin extent (green), and raised peat bogs (light brown; Van Dinter et al., 2014). Anthropogenic features of youngest 1000 yr were removed from map. Selected dates, relevant archaeological sites mentioned in text, and inferred stages of avulsion are indicated. See Data Repository (text footnote 1) for a full description of age control. UK—United Kingdom; NL—Netherlands. Map in RD projection, EPSG: 28992. GSA Data Repository1). All dates are reported in standard calibrated form peatlands, this caused the Rhine branches to raise their levees by ~1 m in yr BCE/CE with a 1σ range. Based on these dates, we identified the between 1000 and 1 BCE (Fig. 2C) and caused the crevasse splays to grow stages of channel initiation at either end of the branches and the matu- larger and faster than their precursors. This accelerated the initial stages of ration stages after the channels connected (Fig. 1). We correlated these avulsion and thus increased the probability that the crevasse splays could events to human activity using archaeological artifacts that were found develop into an avulsion. The increased sediment load and enhanced con- on the natural levees and on top of the peat. Peatland-surface elevation nection of Rhine channels along the peatlands toward the Meuse outlet before avulsion was reconstructed for each 14C sampled site (including resulted in larger sediment transport toward this estuary (Berendsen and compaction correction—Koster et al., 2016, 2018; see the Data Reposi- Stouthamer, 2000). Furthermore, increased freshwater input triggered tory) to assess the gradient advantage along the new course through the floods, which led to the first stage of tidal area expansion close to the pristine peatlands over the abandoned course. mouth of the estuary around 230 BCE (M0 in Fig. 1; Vos, 2015). UPSTREAM AND DOWNSTREAM ANTHROPOGENIC Upstream Initiation of Crevassing (UI) CONTROLS The parent channel belt for the HIJ and Lek avulsion was the second- In the last millennium BCE, two upstream-induced processes deter- ary Rhine branch that ran along the northern edge of the peatland that had mined the geomorphological setting for our study.

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